The present invention relates to a starting control circuit for synchronous motors and, in particular, to a circuit for controlling the application of a DC excitation to the field windings of a synchronous motor to pull the rotor into step with the stator rotating magnetic field.
A synchronous motor includes a field winding which is energized by direct current to magnetize the field poles. In addition to the field winding the rotor also has an amortisseur winding which develops the rotor start-up torque. More particularly, the stator winding is energized by a polyphase power supply to develop a rotating magnetic field. The flux developed by this rotating magnetic field induces voltage and current in the amortisseur winding. The interaction between the generated current and rotating magnetic field develops a starting torque tending to turn the rotor in the same direction as the rotating magnetic field.
An alternating current is also induced in the field winding and is shorted through a field discharge resistor. The induced field current initially has a frequency corresponding to the frequency of the stator supply, and the frequency decreases as the motor approaches synchronous speed. The DC excitation is applied to the field winding as the rotor approaches synchronous speed to pull the rotor into synchronism with the rotating stator magnetic field with minimum rotor slip.
In the prior art, the motor field winding was excited with constant DC current at speeds greater than ninety percent of the motor synchronous speed. If the rotor was not pulled into synchronism upon the application of the constant DC excitation the rotor would decelerate. In applications involving high inertia loads the necessary pull-in torque is great if the motor is to be synchronized by the application of a constant DC excitation. Thus to generate such high torque, oversized machines were necessary.
The present invention solves this problem associated with the prior art synchronous motor controls in that it provides a pulsed DC excitation of the field windings as the induced field current passes through zero followed by removal of the DC excitation at some motor load angle prior to the next subsequent zero crossing of the induced field current. The DC excitation is applied in a polarity supporting the induced field current generating a torque of conventional magnitude which is effective over several cycles of rotor slip in the range of 95-100 percent of synchronous speed. By allowing for synchronism to be achieved over several slip cycles the necessity inherent in the prior art systems of accelerating the load from the point at which DC excitation is applied to 100 percent synchronous speed without slipping a pole is eliminated. Thus large magnitude synchronizing torque is unnecessary and oversized machines are not required.